Today it is under China’s occupation and has been divided up, renamed and incorporated into Chinese provinces (see more maps of Tibet here).
Not until the 18th century did Tibet once again come under a degree of foreign influence.
If Tibet is under unlawful Chinese occupation, Beijing's large-scale transfer of Chinese settlers into Tibet is a serious violation of the fourth Geneva Convention of 1949, which prohibits the transfer of civilian population into occupied territory.
If Tibet is under unlawful Chinese occupation, China's illegal presence in the country is a legitimate object of international concern.
This means that, whereas an independent state that has existed for centuries, such as Tibet, does not need to prove its continued independence when challenged, a foreign state claiming sovereign rights over it needs to prove those rights by showing at what precise moment and by what legal means they were acquired.
China's present claim to Tibet is based entirely on the influence the Mongol and Manchu emperors exercised over Tibet in the 13th and 18th centuries, respectively.